1. Field of the Invention
The present invention relates to a high electron mobility transistor (HEMT), in particular, a HEMT primarily made of nitride semiconductor materials.
2. Background Arts
A conventionally developed HEMT made of nitride semiconductor materials often includes an undoped channel layer made of gallium nitride (GaN), an n-type barrier layer made of aluminum gallium nitride (AlGaN), and an n-type cap layer made of indium aluminum gallium nitride (InAlGaN). Such a conventional HEMT may further provide, on the n-type InAlGaN cap layer, a source electrode and a gate electrode. The n-type InAlGaN cap layer is partially removed to expose a surface of the n-type AlGaN barrier layer, and a gate electrode is provided on the n-type AlGaN barrier layer exposed by the n-type InAlGaN layer.
As well known, nitride semiconductor materials may realize a transistor having an extremely high breakdown voltage because of a wide gap characteristic inherently attributed to those nitride semiconductor materials. Also, a transistor made of nitride semiconductor materials may show good performance in high frequency regions. In order to further enhance the high frequency performance of such a transistor, which is equivalent to increase a cut-off frequency ft of the transistor, the trans-conductance gm of the transistor is necessary to be increased in addition to reduce gate capacitance thereof. Thinner barrier layer may effectively enhance the cut-off frequency ft. A semiconductor material of indium aluminum nitride (InAlN) may be frequently selected as the barrier layer because InAlN may induce an enough electron concentration in the channel layer without increasing a thickness thereof. Reduction of access resistance between the gate electrode and the source electrode of the transistor may also increase the trans-conductance gm of the transistor. The access resistance is a sum of the resistance of the channel between the electrodes and the contact resistance to the electrodes. The reduction of the contact resistance to the source electrode may effectively increase the trans-conductance gm of the transistor. However, when a transistor provides a contact layer on the barrier layer, where the contact layer is generally heavily doped to reduce the contact resistance; inherent access resistance from the electrode to the channel increases because the heavily doped layer and the barrier layer form a barrier in the energy bandgap diagram, namely the band discontinuity, in the interface therebetween.